The present invention relates to control of a nut runner for tightening a screw, and more particularly to a nut runner control circuit for tightening a screw in a plastic region.
In recent mass production of industrial products there has come into what is called plastic region screw tightening according to which a screw is turned by a predetermined angle after the torque characteristic of a nut runner has shifted from an elastic region to a plastic region. However, since it is difficult to accurately detect a critical point from the elastic region to the plastic region, the screw tightening may sometimes take place with a torque smaller than a normal torque for desired plastic region tightening.